In wireless networks, such as wireless cellular networks, digitized speech is carried between a mobile and a base station in coded form to minimise the bandwidth of the transmitted signal. FIG. 1 shows a typical system with a first wireless network 10, a second wireless network 20 and a transmission network 30, such as the PSTN, which connects the wireless networks 10, 20. Considering the path of a signal originating at wireless terminal 11, speech is digitised and coded for transmission over the air interface in the first network 10. A transcoding device (transcoder and rate adaptor unit, TRAU) transcodes the voice data to a conventional PCM 64 Kbit/s format for transmission across the network 30. If, as shown in FIG. 1, the far end of the connection is another wireless network, then the PCM speech signal is again converted from PCM format to a format for transmission over the far end air interface. Each stage of transcoding degrades the quality of speech, and is particularly noticeable when the voice codecs operate at low bit rates.
Tandem Free Operation (TFO) is a known way of carrying speech data across a network where two wireless networks are connected together with a 64 kbit/s transmission path. With TFO, voice data is carried from one wireless party to another, right across any interconnecting network, in the coded form used on the air interface. As shown in FIG. 2, the transcoding stages of devices 15, 25 are bypassed. In this way, the speech is encoded at the transmitting terminal and decoded at the receiving terminal with no intermediate transcoding operations. In order to establish TFO, each wireless network will send out probing TFO in-band signalling (IS) messages to determine the presence and compatibility of the other wireless network. The TFO (IS) messages are in-band messages, embedded within the stream of PCM voice data. If compatible equipment is present, TFO is activated and the wireless networks will exchange TFO frames containing the coded speech data. TFO behaviour is defined in 3GPP TS 28.062 V5.3.0 (2002-12) for GSM/UMTS and in 3GPP2 A.S0004.B v2 (2002-08) for CDMA.
For TFO to operate, any In Path Equipment (IPE) on the 64 kbit/s circuit interconnecting the wireless networks must transparently pass the in-band TFO data. TFO IPE behaviour is defined in Annex B of 3GPP TS 28.062 V5.3.0 (2002-12). As telephony networks evolve, the IPE may comprise a packet based network, such as a network based on Internet Protocol (IP) or Asynchronous Transfer Mode (ATM) protocols. As shown in FIG. 3, a typical packet network has a gateway at each end. While these networks can be transparent to certain signals, they may not allow the TFO data to pass transparently. To understand why this can occur, TFO (IS) messages operate by ‘stealing’ the least significant bit (LSB) of every sixteenth PCM speech sample, while TFO frames steal the n LSBs in every sample. The packet network may itself code (compress) data as part of the packetising process. Other voice processing commonly found in packet networks includes echo cancellation, G.711 log-law coding, silence suppression and DTMF digit relay. This additional compression and processing functions can corrupt the bits representing the TFO (IS) messages.
One solution that has been proposed is for the ingress media gateway, responsible for TDM to packet conversion, to detect TFO (IS) messages or frames and to switch into a clear channel (uncompressed, 64 kbit/s) mode of operation. Operating in this manner has a number of disadvantages. Most significantly, it is wasteful of bandwidth on the packet network. The presence of TFO (IS) messages in a signal does not mean that the call will necessarily proceed to operate in TFO mode. For example, one end of the call may not be a wireless network. Switching to clear channel operation as soon as TFO (IS) messages are detected means that the clear channel transport mode is used on all calls where one end is on a TFO-enabled wireless network, even if the call can not benefit from the TFO voice quality improvement. Further disadvantages of operating in this way are that speech processing (e.g. echo cancellation, A/μ log law conversion) may be disabled, reducing voice quality, and the gateway may introduce audible defects when switching between compression codec and clear channel operation. Furthermore, those calls which do proceed to operate in TFO mode will typically only use 8 or 16 kbit/s of the 64 kbit/s channel allocated to them.
WO01/05113 describes tandem free operation in a system which includes a packet network. Information about decoding capabilities is sent from one gateway to another across the packet network. However, no information is provided as to how the gateways to the packet network can remain transparent to TFO (IS) messages.
Accordingly, the present invention seeks to improve the way in which packet networks handle TFO information.